The invention relates to caching in memory controller hubs.
Microcomputer systems generally include one or more memory controller hubs that control and coordinate the transfer of data between the computer's system memory, central processing unit (CPU), and peripheral devices. Graphics applications may be supported by peripheral devices known as graphics controllers that require a memory controller hub to transfer data between the graphics controller, the system memory, and the CPU.
A design concern associated with microcomputer systems is the quality of two-dimensional (2D), three-dimensional (3D), and video image (collectively referred to below as “graphics”) processing. High-performance graphics processing requires processor-intensive calculations and the fast manipulation of large quantities of data. Several designs have been implemented to achieve high-performance graphics processing while also reducing the cost of the complete system and allowing for upgrades to the computer system's capability.
A computer system may include a graphics controller coupled to local memory for storing graphics data, so that the amount of data that must be transferred between the graphics controller and the system memory and/or the CPU is reduced. Increasing the amount of local memory available to the graphics controller improves graphics performance, but also increases the cost of the computer system, because local graphics memory is relatively expensive. Less local memory is required to achieve the same graphics performance, however, if a dedicated bus, e.g., an Accelerated Graphics Port (AGP), is used to couple the controller to the memory controller hub. An AGP allows the controller to treat portions of system memory as dedicated local graphics memory, which reduces the amount of local memory required and lowers overall system costs.
Computer system costs also may be reduced by eliminating the peripheral graphics controller and integrating its functionality into the memory controller hub. In such a configuration the memory controller hub is better described as a graphics/memory controller hub, since it performs graphics processing functions in addition to memory control and transfer functions. Additionally, it includes one or more output ports to send graphics signals to external devices, such as cathode ray tubes (CRTs) and flat-panel monitors. A graphics/memory controller hub may be coupled to local memory for storing graphics data.